Element Interaction Matrix – New & Enhanced method for Clash Detection

This article discusses the Element Interaction Matrix which measures and evaluates the combination of any two components from various construction trades and defines their interfacing in terms of relevance, severity, and rigidity of components. It also differentiates the interfacing in terms of physical collision or required clearance.

Why do we need a matrix in the first place?

Clash detection is now a word which is synonymous with the BIM environment. In fact, many people relate BIM to clash detection only, and for obvious reasons - never before they were able to see coordination issues so clear in crisp 3D environment.

In the BIM Environment, Clash detection ability is a powerful tool provided by many software which helps in identifying all instances where objects are colliding with each other. But this comes with a problem – all colliding objects may not necessarily be a clash. At design stage coordination check, all pipes passing though slab are picked up as clashes. Similarly, electrical fixtures, security devices which are fixed on walls or ceilings are picked up as clashes. These are either to be ignored or should not be considered for clash test in the first place.

Therefore clash-detection requires a thorough strategy where factors such as clash policies, severity levels, clearance and hard clashes are taken into account. Autodesk prepared and proposed a clash detection matrix in 2015 for Qatar Rail. This matrix depended upon the extra parameters which were required to be incorporated into the models for each element. This limited the capability of BIM professionals to use this matrix as authoring of models is not always in their control, where they could enforce the creation of additional parameters for clash detection. The proposed matrix also lacked clarity in identifying the components where clearance was required. Nevertheless, this matrix provided a solid basis for further research and expansion.

This Author has worked on the TOD projects for Qatar Rail and having understood the limitations in the Autodesk clash matrix, he reworked the matrix in conjunction with the site engineers and architects. In this exercise each combination of elements was studied and viewed in wider perspectives which included actual site conditions, complexity, design standards and conventions to understand the below aspects;

a.??????Possibility of clash

b.??????Relevance of clash

c.??????Rigidity of components

d.??????Severity of clash

e.??????Nature of clash (Hard or Clearance)

f.???????In case of Clearance test, min clearance requirement between two objects

g.??????Tolerance values

h.??????Exceptions

The below sections further elaborate and analyze the above listed aspects which form the basis of development of the Matrix.

a.??????Possibility of Clash - This article mainly focuses on the models received/delivered for purpose of construction which is also referred to as “Issued for Construction” (IFC). Therefore, we try to understand the ‘possibility of clash’ from the context that the models have crossed the basic authoring stage and are mature enough to be used for Construction.

This aspect of ‘possibility’ is important to understand as not all components have direct interfacing with each other. For example, railings & ceiling do not have any natural direct interfacing. BIM Authoring platforms generally do not allow placement of railings on the ceiling. Any clash occuring between these two items would generally mean a modelling error. If any rare occurrence of such errors exists in the project, then these are picked up by the modelers at the IFC stage and get resolved. Therefore, in the Clash Matrix, such combinations shall be marked as ‘N’, implying no test is required.

b.??????Relevance of Clash – As mentioned earlier in this article that many combinations of elements have naturally colliding interface. Such as Doors & Windows are always hosted on Walls, Light Fixtures and other sensors are usually hosted on ceilings, and if clash test is run between these elements, it will return many clashes which hold no relevance. Therefore, such combinations in the matrix shall also be marked as ‘N’, implying no test is required.

c. Rigidity of Components – Rigidity of the components is measured in terms of their?criticality in the design. Such as spaces are the main aspects of design represented by Walls. These walls could be architectural or structural in nature but in both cases, they represent space. Therefore, these are ‘Rigid’ components. Using the same approach of criticality, Architectural Railings or Mechanical Equipment offer some flexibility in their placement. These are classified under ‘Semi-Rigid’ components, whereas most of the pipes, sprinklers, light fixtures often provide full flexibility in terms of their placement, therefore these can be placed under the ‘Flexible’ category.

d.??????Severity of clash – This is usually determined by establishing a rule of thumb for interacting components. As shown in the table above, combination of the three ‘Rigidity’ levels produces six combinations. These 6 combinations can either be used in the matrix as 6 severity levels using different color coding for each. Alternatively, these can be narrowed down to three broad categories of high-medium-low as the Author has categorized in the table below for a project specific matrix. Severity Levels also determine the impact on design, cost and on construction schedule of the project. ?The below table provides the coding structure for severity levels and for naming convention of Clash-Sets in the Navisworks program.

If the names of all high priority tests start with the string ‘A’, then all high priority tests will appear on top of the list in Navisworks Clash Detection module, thus making it visually easy for BIM coordinators to focus as per priority.?

e.??????Nature of clash (Hard or Clearance) - Most of the time BIM professionals focus on Hard-Clashes only where items are found physically colliding with each other. But equally important and in some cases critical is the ‘Clearance Test’ where the idea is to check if the minimum required clearance between two objects is available. If not, then such instances are also called ‘soft clashes.’ Clearance test returns more results than the Hard test, as it not only picks up the soft clashes, but also the hard ones. These clashes shall also be pre-classified as per the above severity levels. In the matrix, these are simply represented as ‘H’ for hard test or ‘C’ for Clearance test. Other cells are populated as ‘N’ as per sections (a) and (b) above.?

f.??????Clearance test, min clearance requirement – This shall be done in conjunction with the site team or design & maintenance engineers. Each combination in the matrix is studied carefully to determine if it requires any clearance between the elements. For example, Busbars transfer high voltage current from one place to other. Because of high voltage, it creates a magnetic field around it. Any ELV containment or IT fixture shall not be in the close vicinity of the Busbars as magnetic field may impact the functionality of these items. Therefore, all such combinations are identified in the clash matrix and reasonable clearance value is shown in the cell. For many combinations, clearance is required only for maintenance purposes.

g.?????Tolerance values – Many combinations for hard tests may be given certain tolerance based on their severity category. For high priority clash tests such as air terminals and steel framing, no tolerance can be allowed as these items should remain clear of each other. Whereas for other low priority combinations such as Electrical Fixtures & Pipes, clash tests may be allowed to have certain tolerance e.g 25mm as both these items are flexible and can be moved on site without being reported as clash.?

h.??????Exceptions – For certain high priority clash tests, high tolerance needs to be allowed-for to avoid detecting clashes. For example, all ducts and pipes which are fitted to the underside of staircase shall not be picked up as clashes, but at the same time any duct or pipe penetrating through the staircase is a serious clash. Pipe fixing clamps or hangers penetrate in the slab for up to 50mm, but these items are not generally modeled at LOD 300 or 350, so modelers at the design stage show these pipes directly attached to the underside of the stair slab. If the tolerance of 50mm is allowed for such combinations, then such collisions will not be picked up as clash, on the other hand any pipe or duct clashing with or penetrating the stair slab for more than 50 mm then it will be picked up in the same test.?

After understanding and establishing the above eight (8) aspects, a matrix can be created for each combination of components. It requires due diligence, patience, discussions with colleagues and lots of time to compile this information in the form of a matrix. The author prefers to call this as ‘Element Interaction Matrix’ (EIM) instead of clash matrix as this shall be prepared prior to model finalization and shall be used by the engineers and modelers from the onset of the project modelling. ?

In the absence of any such matrix, many BIM professionals perform clash-detection with everything against everything resulting in huge numbers of clashes which act as deterring factor to move to next step of analyzing and assigning the clashes to responsible disciplines. Author on his current project witnessed the client-side BIM Manager forcing the contractor to perform clash test without any matrix, resulting in huge man-hour wastage in sorting and analyzing thousands of clashes at each submission of models.

Inspired by this mal-practice and as a service to the construction industry, the Author allows the EIM to be used in the projects wherever needed and can be requested through an email.

This EIM provides the numbering system, coding structure of the components and naming convention for the clash sets. It also lists all the exceptions adopted for some combinations.

The Author invites further suggestions from the construction fraternity to help improve the matrix, while keeping in mind the fact that the more it gets complex, less is the probability of its full adoption.

Thus, the Author hopes that with the help of the provided EIM, BIM professionals will not fall pray of ‘everything against everything’ and perform modeling and clash detection tasks with reasoning and confidence.?